1. Field of the Invention
The present invention relates to a signal strength detecting device mounted on a radio communications apparatus for detecting the signal strength of a signal received by an antenna.
2. Description of Related Art
A large number of radio communications apparatuses incorporate a signal strength detecting device for detecting the signal strength of a signal received by an antenna.
In particular, in a mobile communications system employing a CDMA (Code Division Multiple Access) scheme, a base station and a mobile station each detect the signal strength of a signal transmitted from their party, so as to precisely control the strength of a signal to be transmitted to the party in accordance with the signal strength.
For example, when the base station and mobile station are far away from each other, since the signal strength of the received signal is weak, the strength of the transmitted signal is increased so that the party can receive the transmitted signal with sufficient intensity.
In contrast with this, when the base station and mobile station are close to each other, since the signal strength of the received signal is strong, the strength of the transmitted signal is reduced so that it does not interfere radio signals from other base stations or mobile stations.
FIG. 6 is a circuit diagram showing a conventional signal strength detecting device. In FIG. 6, the reference numeral 1 designates a logarithmic amplifier for amplifying a signal received by an antenna; 2 designates an input terminal for supplying the signal received by the antenna; 3-5 each designate a differential amplifier for amplifying the received signal supplied from the input terminal 2; 6-8 each designate a transistor constituting the differential amplifiers 3-5; 9-11 each designates an emitter follower for regulating the DC level of the received signal amplified by the differential amplifiers 3-5; and 12 designates a current source for biasing the logarithmic amplifier 1. The current source 12 is a constant current source whose current is proportional to the absolute temperature.
The reference numeral 21 designates an amplitude detector for detecting the signal strength of the received signal by comparing the received signal whose DC level is regulated by the emitter followers 9-11 of the logarithmic amplifier 1 with a reference voltage; 22 designates a reference voltage generator for generating the fixed reference voltage; 23-25 each designate a differential amplifier for comparing the received signal whose DC level is regulated by one of the emitter followers 9-11 of the logarithmic amplifier 1 with the reference voltage generated by the reference voltage generator 22, and for producing a collector current proportional to the difference between the two; 26-28 each designate a transistor constituting the differential amplifiers 23-25; 29 designates a capacitor for smoothing the collector current output from the differential amplifiers 23-25; 30 designate a PNP current mirror circuit for inverting the polarity of the collector current smoothed by the capacitor 29; 31 designates a resistor for converting the collector current to a voltage signal; 32 designates an output terminal for producing the voltage signal indicating the signal strength of the received signal; and 33 designates a current source for biasing the amplitude detector 21. The current source 33 is a fixed current source whose current is proportional to the absolute temperature.
Next, the operation of the conventional signal strength detecting device will be described.
First, when the signal received by the antenna is supplied to the input terminal 2, the differential amplifier 3 of the logarithmic amplifier 1 amplifies the received signal.
The received signal amplified by the differential amplifier 3 is output from the collectors of the transistors 6 and 7. The received signal output from the collector of the transistor 6 is subjected to the DC component regulation by the emitter follower 9, and then supplied to the differential amplifier 23 of the amplitude detector 21 as well as to the transistor 6 of the post-stage differential amplifier 4. On the other hand, the received signal output from the collector of the transistor 7 is supplied to the transistor 7 of the post-stage differential amplifier 4.
Receiving the received signal output from the differential amplifier 3, the differential amplifier 4 of the logarithmic amplifier 1 further amplifies the received signal.
The received signal amplified by the differential amplifier 4 is output from the collectors of the transistors 6 and 7. The received signal output from the collector of the transistor 6 is subjected to the DC component regulation by the emitter follower 10, and then supplied to the differential amplifier 24 in the amplitude detector 21 as well as to the transistor 6 in the post-stage differential amplifier 5. On the other hand, the received signal output from the collector of the transistor 7 is supplied to the transistor 7 in the post-stage differential amplifier 5.
Receiving the received signal output from the differential amplifier 4, the differential amplifier 5 in the logarithmic amplifier 1 further amplifies the received signal.
The received signal amplified by the differential amplifier 5 is output from the collector of the transistor 6. The received signal output from the collector of the transistor 6 is subjected to the DC current regulation by the emitter follower 11, and then supplied to the differential amplifier 25 in the amplitude detector 21.
Receiving the received signal amplified by the differential amplifiers 3-5 in the logarithmic amplifier 1, the differential amplifiers 23-25 in the amplitude detector 21 compare the received signal with the reference voltage generated by the reference voltage generator 22, and output the collector currents Ic proportional to the difference between the two.
The collector currents Ic output from the differential amplifiers 23-25 are smoothed by the capacitor 29, undergo the polarity inversion by the PNP current mirror circuit 30, and are converted by the resistor 31 to a voltage signal (that indicates the signal strength of the received signal) to be output from the output terminal 32.
Here, the reason will be described for using the fixed current source proportional to the absolute temperature as the current source 12 for biasing the logarithmic amplifier 1.
First, the gain characteristic of the differential amplifiers 3-5 in the logarithmic amplifier 1 is given by the following expression.
G=(qI/kT)RL
where G is the gain, q is the elementary charge, I is the magnitude of the collector current, k is the Boltzmann constant, T is the absolute temperature, and RL is a load resistance.
If the logarithmic amplifier 1 is biased by a current source without temperature dependence, the collector current I will be fixed, and the gain G of the differential amplifiers 3-5 will be reduced in inverse proportion to the absolute temperature T.
However, this will reduce the gain G of the differential amplifiers 3-5, making it impossible to amplify the received signal to a desired level. Accordingly, the collector current I must be made proportional to the absolute temperature T.
In view of this, the fixed current source with temperature dependence is used to provide the logarithmic amplifier 1 with the bias current proportional to the absolute temperature T.
Thus, the conventional signal strength detecting device with the foregoing configuration can amplify the received signal by means of the logarithmic amplifier 1 without being affected by temperature change. However, since the current source 33 for biasing the amplitude detector 21 also consists of the constant current source whose current is proportional to the absolute temperature T, even if the signal strength of the received signal is kept constant, the change in the absolute temperature T will vary the collector current Ic output from the differential amplifiers 23-25. This presents a problem of hindering accurate detection of the signal strength of the received signal.
The present invention is implemented to solve the foregoing problem. It is therefore an object of the present invention to provide a signal strength detecting device capable of detecting the signal strength of a received signal accurately without being affected by the temperature change.
According to one aspect of the present invention, there is provided signal strength detecting device comprising: an amplifier for amplifying a received signal received by an antenna; a strength detector for detecting signal strength of the received signal by comparing the received signal amplified by the amplifier with a reference voltage; a first current source for biasing the amplifier; and a second current source for biasing the strength detector, wherein temperature characteristic of the second current source differs from that of the first current source.
Here, the first current source may consist of a fixed current source whose current is proportional to absolute temperature, and the second current source may consist of a constant current source whose current is not proportional to the absolute temperature.
The amplifier may comprise differential amplifiers, at least one of two outputs of each of the differential amplifiers being connected to an emitter follower.
The amplifier may comprise differential amplifiers, at least one of two outputs of an initial stage of the differential amplifiers being connected to an emitter follower.